1. Field of the Invention
The invention relates generally to a method and apparatus for rotary actuator arc compensation correction in a direct access storage device (DASD).
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.
All DASD units must have a method to position each data head over the proper radial location to write a track and again, to position it very close to the same location to read the track. With the higher level files using a voice coil type of actuator, a feedback mechanism must be provided to locate and stably hold the head on a given track. Typically, track accessing and track following is provided utilizing a magnetically written pattern in the DASD unit. A dedicated servo system employs one surface of one of the disks in the DASD on which to have all the tracking and access information. A sector servo system uses small portions of tracks between each or between several sectors on each track of each data surface to provide the tracking and access information. A hybrid servo system uses both to obtain advantages of each type of servo.
The degree of tracking accuracy, i.e., the ability of the actuator servo to keep the head position discretely over the track, is governed by two factors. One factor is spacial (space), the other is temporal (time). The spacial factor corresponds to the number of servo sectors N around the track, which is a function of the linear recording density and the fixed block data format. The temporal factor corresponds to the time between servo sectors or the sampling period, which is controlled by the rotational disk velocity (RPM).
Track misregistration (TMR) error can be separated into two major components, repeatable or synchronous with disk rotation and non-repeatable or asynchronous with disk rotation. The repeatable TMR component, which can be large in case of a disk slip, can be reduced by correction compensation. A correction for radial track misregistration (TMR) versus radius is required for a DASD with a rotary actuator when tracks are written approximately uniformly on the rotary arc rather than in the radial direction. Both the gain and phase effects with radius should be compensated. Both gain and phase effects with radius produce comparable amounts of TMR error in the system.
In DASD with a rotary actuator, the head does not move in a radial direction. For many disk drives, the skew angle of the head has the back end of the head further out on the disk than the front when the head is at the inner radius. The skew gets progressively larger at larger radii. This is used to advantage in the drives by writing the tracks nearly uniformly on the arc that the head transverses from the inner radius to the outer radius. Due to the larger skew angles, the written tracks are physically smaller near the outer radius than near the inner radius. The positioning of the tracks on the arc causes the radial track pitch to be smaller at the outer radii than at inner radii, which is consistent with the smaller written tracks.
A radial movement of a disk, such as that due to disk slip, or other vibrations producing an apparent out-of-round for the track, moves the disk the same physical distance at any radius. However, in the servo system, the distance is measured in fractions of a pitch. Typically this distance is in 1/512 of a customer track pitch. Since the head is moved at the skew angle for a given radius, the head must be moved further than the radial movement of the disk, and this amount is more significant at the outer radii where the skew angle is larger than at inner radii.
As a result, any compensation for radial TMR that is determined at one radius in a given number of units in the servo system will not be the same value at other radii. For example, a difference between the outside diameter (OD) and the inside diameter (ID) can be about 7%. At progressively higher track density this has some significance as an error in the corrections. Also, when a prewritten servo disk is used, there can be significant repeatable runout, thus increasing the magnitude of the error.
In known disk files which used dedicated servo control, it was necessary to add a reference profile to correct for runout due to disk slip or relative thermal motion between each head and the corresponding tracks on the data surface. These reference profiles, measured for each data surface, would have been correct if a linear actuator was used, since a linear actuator will normally follow a radial trajectory on a disk. On the other hand, a rotary actuator follows an arc rather than a radial line on the disk, thus there will be a phase error between the track where the runout was measured and any other track.
Known feedforward or profile compensation techniques used for rotary actuators to correct for runout or disk slip have a systematic error, for example, such as, up to 10% of the runout for 3.5" DASD caused by the arc made by the product head. A disk slip and/or imbalance will occur in a radial direction, and not along the head arc, thus the measured profile will only be correct at the track or cylinder of measurement. All other tracks will have an incorrect profile for optimal runout compensation.